The invention relates to a semiconductor laser of the surface-emitting type having an active zone with a pn transition (pn junction).
Semiconductor lasers are in principle semiconductor diodes operated in the direction of flow which generate coherent light of a narrow spectral band by stimulated emission and shine it in a directed manner. The population inversion needed for the laser process is achieved by an injection of current into the pn transition. As a supporting measure the starting material may be highly doped. In the region of the pn transition in which the electrons and holes are adjacent to one another in space, the induced radiant recombination then takes place.
The optical resonator is formed, in the simplest case, from two opposing optical mirrors at right angles to the pn transition. In this construction, emission takes place in the plane perpendicular to the injection of current.
Alternatively, surface-emitting semiconductor laser diodes are already known in which emission takes place perpendicularly to the plane of the active zone. (i.e., vertical-cavity surface-emitting laser diode, VCSEL).
From U.S. Pat. No. 5,936,266 (issued Aug. 10, 1999), a semiconductor laser of the surface emitting type is known in which tunnel contact over the entire surface is used to create a conductive transition between the p side of the active zone and an n-doped semiconductor layer. This means that n-doped semiconductor layers can also be used on the p side of the active zone, resulting in electrical series resistances which are 10–30 times lower owing to the improved electrical conductivity of n-doped semiconductors. However, one disadvantage of tunnel contact over the whole surface is that additional oxide layers have to be provided for controlled guidance of the current, which result in a complex and thermally unfavourable structure of the semiconductor laser as a whole.
U.S. Pat. No. 6,052,398 (issued Apr. 18, 2000; the “'398 patent”) describes a semiconductor laser of the surface emitting type which has a structured tunnel contact, the resonator being formed by two semiconductor mirrors. The problem here is that the heat has to be removed through one of the mirrors, which usually consist of ternary or quaternary mixed crystals with correspondingly poor thermal conductivity. Alternatively, the '398 patent also mentions the use of a dielectric mirror on the p side of the active zone without mentioning any particular advantages attaching to this alternative solution. In practice, this solution is not used because reflective contact layers (usually gold or silver) may diffuse into the adjacent semiconductor layers, with the result that long term stability cannot be guaranteed.